The present invention concerns a blanket designed for a printing machine having a gap. Such a blanket is classically held on the cylinder, its edges of the free extremity being attached to the interior of the gap.
The presence of this gap entail inconveniences which are well known to the specialist.
First of all, the presence of such a gap induces mechanical vibrations of the cylinder which generate variations in pressure in the nipping zone. These variations affect the quality of printing and the defect known under the name of streaks is very often redhibitory.
Moreover, this defect is the factor limiting the speed of cylinders and thus of the productivity of printing machines.
Another problem associated with rotational offset printing with gap cylinders is the absence of printing at the level of the gap. This constitutes a loss of paper for the printer which can be estimated at about 1.5% of the weight of paper. Then, it is paper which comprises the dominant element in terms of the cost of the printing process.
One can namely cite the document FR-2 660 895 which describes an endless sleeve permitting using gapless cylinders.
This solution is effective but cannot always be utilized. It necessitates in fact a special configuration of printing machines. An endless sleeve can only be introduced axially on a cylinder.
Document FR-2 589 102 for its part describes a device for damping the mechanical vibrations of a cylinder rotating on a gap.
Such a device is effective, but it does not permit reducing the width of the gap, the problem of the loss of paper during printing thus remaining unaffected.
One can furthermore cite document U.S. Pat. No. 5,749,298 which describes a printing blanket called xe2x80x9cmetal-backed,xe2x80x9d which has a stack of layers fixed on an interior metal layer ensuring fixation on the printing cylinder.
The structure of such a blanket is not continuous, the stack of layers stopping at the level of the gap and only the magnetic interior layer penetrating to the interior of the gap.
This blanket thus permits reducing the dimension of the gap and vibrations generated at high speeds of rotation.
Nonetheless, such a printing blanket has a major inconvenience. As a matter of fact, its discontinuity in the gap zone constitutes a mechanically weak point in the structure of the blanket.
U.S. Pat. No. 4,537,129 describes an offset printing blanket which is designed to reduce the width of the cylinder gap on which it is held and thus to increase the number of printed lines on the cylinder circumference.
This blanket is constructed in two parts, a compressible thick layer which stops at the level of the gap and a thin printing layer which penetrates into the gap.
This blanket poses problems of adjustment of the two layers superposed around the cylinder. Moreover, it is solely the upper printing layer which supports the entire tension of the blanket which constitutes a limitation on the mechanical plane.
The invention has for its purpose to palliate these inconveniences by proposing a printing blanket the free extremities of which can be inserted and secured in a narrow gap of a blanket-bearing cylinder to ensure the tension of the blanket, all the while maintaining the integrity of the structure of the blanket in a manner so as to limit the vibrations of the cylinders of the printing machine and thus printing errors and to reduce the loss of paper during printing, the blanket not having a mechanically weak point.
Such a blanket thus permits increasing the productivity of printing machines because they can print at a very high speed without loss of quality while realizing economy in paper.
Generally, the invention consists of locally thinning the blanket at the level of its free extremities which should penetrate into the gap of the cylinder.
The invention first of all concerns a process for obtaining a printing blanket of which at least one of the free extremity edges, designed to be retained in the gap of the blanket-bearing cylinder, is thinner than the rest of the blanket, characterized in that it consists:
In creating first of all a printing blanket of a given thickness, the continuous structure of which includes at least one elastomer-based lithographic or printing layer, a layer of compressible material and a reinforcement layer,
Of applying a pressure on the free extremity edge of the blanket to bring about a heat compression of this edge, a pressure between 10 and 30 MPa and at a temperature between 100 and 250xc2x0 C. so that the thickness of this edge is reduced by at least 25% in relation to the rest of the blanket while preserving the integrity of the structure of the blanket.
Preferably, this pressure is applied on the free extremity edges of the blanket and on about 5 to 15% of the overall surface of the blanket.
Advantageously, the process of the invention likewise consists of rigidifying at least one free extremity edge of the blanket to increase the flexion module of the extremity edge.
The process of the invention then consists of placing, before applying the pressure and on at least one of the extremity edges of the blanket, a rigid material or a material designed to become rigid under the action of heat, such as a strip of metal, of plastic material or even a thermoplastic or heat-hardening resin, this material being incorporated into the blanket by the heat compression.
The process of the invention likewise advantageously consists of providing a layer of rigid material on the back of the blanket, comprised by a resin in the form of a liquid or gel, overlaid or impregnated on the back of the blanket, or by a reinforcing film, namely of polyamide, polycarbonate or polyolefin type, and to fix this layer of rigid material on the blanket during the application of heat pressure to increase the traction module of the blanket.
Preferably, mineral or organic fillers, or even fibers and filaments are added to the resin to improve the mechanical properties of the blanket.
Preferably, the process of the invention likewise consists of fashioning at least one free extremity edge of the blanket, imparting to the leading edge an angle falling between 15 and 120xc2x0 in relation to the printing surface, and giving the trailing edge an angle between 60 and 165xc2x0 in relation to the printing surface.
The process advantageously consists, after the heat compression operation, of allowing the blanket to cool under pressure to preserve the reduction in thickness of the extremity edge.
The invention likewise concerns a mold for implementing the process of the invention.
This mold is characterized in that it includes two parts, the first of which is perceptibly flat and the second of which is comprised by a plate containing at least one element, such as a bar or something analogous offset in relation to the first part capable of realizing the operation of heat pressure against the first part on a free extremity edge of a printing blanket.
Preferably, the second part includes an internal face in the extension of the element or elements which is designed come in support on the blanket during heat pressure to avoid creep and reduce the concentration of stresses.
In a manner to be able to rigidify at least one free extremity edge of the blanket, the mold of the invention includes, in at least one element, a recess for placing a rigid material, such as a reinforcement strip of metal, or of plastic material.
The invention likewise concerns a printing blanket including at least one relatively thin free extremity edge including at least one elastomer-based lithographic or printing layer, a compressible layer and a reinforcement layer, characterized in that this wire or fiber-based reinforcement layer presents a cover factor between 75 and 96%, preferably between 75 and 88%, as well as a free volume greater than 55%, preferably greater than 64%, this blanket having a thickness reduced by at least 25% in relation to the rest of the blanket on a surface extending on at least one free extremity edge of the blanket and including between 5 and 15% of the overall surface of the blanket.
Preferably, the printing blanket of the invention includes at least one thermoplastic elastomer layer, namely of polyurethane, polyolefm, polystyrene, polyamide, polyester or polyacrylic type, or even a mixture of such polymers.
In this case, the blanket of the invention advantageously includes a reinforcing layer presenting a relatively significant rigidity in the direction of travel, that is to say, parallel to cylinder gap on which the blanket is designed to be mounted.
The blanket of the invention can likewise include at least one elastomer layer the acrylonitrile content of which is at least 16%, the lithographic or printing layer advantageously being of this type.
The blanket of the invention advantageously includes a fabric or mesh presenting a high traction module.
The blanket of the invention likewise advantageously includes a layer of rigid material on the back of the blanket to augment the traction module of the blanket, this layer being namely constituted by a layer of hardened resin or a film, for example, of polyamide.
Preferably, at least one of the free extremity edges of the blank of the invention includes a rigid material incorporated into the blanket, of a strip type of metal, plastic material or even hardened resin to augment the flexion module of the extremity edge and to serve as a means of securing the blanket on the cylinder.
The printing blanket of the invention advantageously includes at least one preformed free extremity edge, the leading edge forming an angle between 15 and 120xc2x0 in relation to the printing surface, while the trailing edge forms an angle between 60 and 165xc2x0 in relation to the printing surface.